1. Field of the Invention
The present invention relates, generally, to an anti-lock control system and, more particularly, to an anti-lock control system wherein a pressure increasing cycle changes according to the wheel speed, during braking of the vehicle.
2. Discussion of the Prior Art
Typical anti-lock control systems for motor vehicles include a control unit having a microcomputer operating as follows: A wheel speed is detected by a wheel speed sensor, a controlled wheel speed (hereinafter referred to as "a control speed") is selected according to an electrical signal representing the detected wheel speed and a brake fluid pressure control mode, for increasing, decreasing or maintaining a brake fluid pressure, is determined according to the control speed selected. A hold valve comprising a normally opened electromagnetic valve and a decay valve comprising a normally closed electromagnetic valve are operated (opened and closed) so that the brake liquid pressure is increased, decreased or maintained constant.
When the hold valve is opened and the decay valve is closed, the brake fluid pressure is increased. When the hold valve is closed and the decay valve is also closed, the brake fluid pressure is maintained. When the hold valve is closed and the decay valve is open, the brake fluid pressure is decreased. During a normal braking, the brake fluid pressure increases and held cyclically (pressure increasing cycle) so that the brake fluid pressure is increased stepwise. In this case, the period of time the hold valve is closed is held constant and the period of time the hold valve is open is changed, thereby changing the duration of the pressure increasing cycle.
In conventional anti-lock control systems, the duration of the pressure increasing cycle is determined from the coefficient of friction .mu. of a road surface (hereinafter referred to as "a road surface .mu.") which is detected before the pressure increasing mode occurs, thereby predetermining a pressure increasing cycle. In the pressure increasing mode, the brake fluid pressure is increased according to the pressure increasing cycle thus set. This means that the pressure is increased irrespective of the behavior of the wheels. Hence, if the road surface .mu. decreases during the pressure increasing operation, the pressure is excessively increased so that the wheel speed is greatly reduced. Whereas if the road surface .mu. then increases, the pressure becomes insufficient, and the braking force is not great enough.